1. Field of the Invention
The present invention relates to a developing unit for use in an image forming apparatus utilizing an electrophotographic system, such a copier, a printer, a facsimile machine and a complex machine thereof, and an image forming apparatus equipped with the developing unit.
2. Description of the Related Art
A development unit for use in an image forming apparatus utilizing an electrophotographic system is designed to allow toner carried by a developing roller to fly toward a surface of a photoconductive drum which is an image support member having thereon an electrostatic latent image formed thereon in conformity to image data, so as to form a toner image. In the image forming apparatus equipped with such a developing unit, the toner image formed on the photoconductive drum is transferred onto a recording medium, such as a sheet. Then, the transferred toner image is heated and fixed on the recording medium by a fixing unit. In this manner, the image forming apparatus can form an image conforming to the image data, on the recording medium.
Among various types of the above image forming apparatuses, a tandem-type image forming apparatus is known as one having excellent high-speed performance, wherein a plurality of image forming units corresponding to respective colors of toner to be used are arranged in a line, in such a manner as to perform a color superimposition process on an intermediate transfer belt while forming each color image in synchronization with a feed rate of the intermediate transfer belt. Although the tandem-type image forming apparatus surely has excellent high-speed performance, it has a problem about an increase in size due to the need for the in-line arrangement of the plurality of image forming units for respective colors. As one of the measures against this problem, there has been proposed a small-sized tandem-type image forming apparatus designed to narrow a distance between adjacent ones of a plurality of photoconductive drums and correspondingly arrange a plurality of downsized image forming units. In the small-sized tandem-type image forming apparatus, a fluctuation in movement of a developing roller and a fluctuation in movement of a photoconductive drum become larger due to the downsizing. Generally, the downsizing also causes an increase in a fluctuation in movement of a pulley which is disposed on an outward side relative to a longitudinal end of the developing roller and adapted to be rotationally driven by the photoconductive drum while being kept in contact therewith, so as to define a gap (developing gap) between the photoconductive drum and the developing roller. Moreover, vibration or shaking of the developing roller becomes larger due to an increase in rotational speed thereof to accelerate the fluctuation in movement of the pulley. Consequently, in the small-sized tandem-type image forming apparatus, the developing gap is liable to be largely fluctuated.
The above developing unit includes one type designed such that a bias voltage is applied to a developing roller to cause toner to fly toward a photoconductive drum, based on a developing electric field formed between the developing roller and the photoconductive drum, i.e., by means of so-called “jumping phenomenon”. In this type of developing unit, if a developing gap is fluctuated, the developing electric field will be changed to lead to a change in a toner amount on development, which causes image unevenness.
In view of meeting a recent demand for higher image quality, it is necessary to reduce a fluctuation in image density and image unevenness due to the fluctuation in the developing gap. Particularly, in the small-sized tandem-type image forming apparatus which has a relatively large fluctuation in the developing gap as described above, there is a strong need for reducing image unevenness due to the developing gap fluctuation.
In the above type of developing unit, image unevenness due to the developing gap fluctuation occurs in both a solid image and a low-gradation image (halftone dot image). The reason would be that an image density of an obtained image is influenced by the developing gap.
FIG. 11 is a graph showing an influence of a developing gap on respective image densities of a solid image and a halftone dot image, wherein the vertical axis represents a reflection image density of an obtained image, and the horizontal axis represents a developing gap (μm). This graph shows a result of a test where an image was formed using a developing bias voltage set as follows: a duty ratio of an AC voltage to be applied to a developing roller in a direction for causing toner to develop an electrostatic latent image=30%; a voltage (maximum voltage) in a direction for causing toner to develop an electrostatic latent image=1000 V; a voltage (minimum voltage) in a direction for pulling toner back to the developing roller=−500 V; and a frequency=2.5 kHz.
As seen in FIG. 11, respective image densities of the solid image and the halftone dot image are lowered in both cases where the developing gap is excessively narrow and excessively wide. Therefore, if the developing gap is fluctuated, the image density will be fluctuated to cause image unevenness. FIG. 11 also shows that the tendency of lowering in the image density is slightly different between the solid image and the halftone dot image.
As above, while both the solid image and the halftone dot image are influenced by the developing gap, a pattern of the influence is different therebetween. Thus, it has been proposed to generally increase a toner amount on development in order to reduce image unevenness due to the developing gap fluctuation. However, if the toner amount on development is increased up to a given value or more, toner scattering during a transfer process, or an adverse effect on a fixing performance is likely to occur.
As a development technique of controlling a transfer bias voltage in the jumping phenomenon, there has been known a technique of applying a blank pulse bias where a ratio of a first duration to form an alternate electric field between a developing roller and a photoconductive drum to a second duration to stop forming the alternate electric field is set in the range of 1:0.5 to 1:10 (see the following Patent Publications 1 and 2). In connection with the above technique of applying the blank pulse bias in the jumping phenomenon, it is also disclosed that a last-stage component of the alternate electric field in the first duration is formed as an electric field component in a direction for pulling toner back to the developing roller (see the following Patent Publication 3). There has also been known an image forming apparatus using a two-component developing system, designed to apply a developing bias including an oscillation period and a pause period, to a developing roller supporting a two-component developing agent containing a toner and a carrier, wherein the developing bias, wherein a first oscillating bias just before transition from the oscillation period to the pause period is set in a direction for causing the toner to fly toward an image support member, and a second oscillation bias just before the first oscillation bias is set in a direction for pulling the toner back to the developing roller (see the following Patent Publication 4).
[Patent Publication 1] JP 02-014704B
[Patent Publication 2] JP 02-014705B
[Patent Publication 3] JP 02-014706B
[Patent Publication 4] JP 2001-194876A
In the conventional developing unit, respective image densities of a solid image and a halftone dot image are lowered in both cases where a developing gap is excessively narrow and excessively wide, as described above. This would be based on the following factors.
In cases where the developing gap is excessively narrow, even if toner is attached to an electrostatic latent image, an electric field formed by an AC voltage in a direction for pulling the toner back to a developing roller becomes excessively strong, and thereby the toner attached to the electrostatic latent image is pulled away from the electrostatic latent image to cause lowering of a toner amount on development.
In cases where the developing gap is excessively wide, an electric field formed by an AC voltage in a direction for causing toner to develop an electrostatic latent image becomes excessively weak, and thereby a toner amount reaching the electrostatic latent image is reduced to cause lowering of the toner amount on development.
The difference in the tendency of lowering in image density between the solid image and the halftone dot image would be based on the following factors.
In the halftone dot image, an electric field to be formed by a difference between respective potentials in a latent image region (i.e., a portion of a photoconductive drum on which an electrostatic latent image is formed) and a non-latent image region (i.e., the remaining portion of the photoconductive drum on which no electrostatic latent image is formed), so-called “cross-talk electric field” arising from the potential in the non-latent image region”, is relatively strong. Thus, the latent image region has a relatively strong toner retaining force. In the solid image, while an edge region has a relatively strong toner retaining force as with the halftone dot image, an inward region other than the edge region has a lower toner retaining force than that in the edge region. Therefore, when the developing gap becomes narrower, the solid image has a tendency to allow toner to be easily pulled away by the AC voltage as compared with the halftone dot image.
Further, as the cross-talk electric field arising from the potential in the non-latent image region becomes weaker, a developing electric field between the developing roller and the photoconductive drum becomes stronger. Thus, the halftone dot image or the edge region of the solid image has a weaker development electric field, and the inward region of the solid image has a stronger development electric field. Therefore, when the developing gap becomes wider, the solid image has a tendency to have a larger amount of toner reaching to the photoconductive drum as compared with the halftone dot image.
The Patent Publication 1 includes a description about an advantage of being able to suppress density unevenness due to non-uniformity in a development magnetic field. The Patent Publication 2 includes a description about an advantage of being able to suppress a negative characteristic that a solid image has a lower density than that of a halftone dot image. The Patent Publication 3 includes a description to the effect that the last-stage component of the alternate electric field in the first duration can be formed as an electric field component in a direction for pulling toner back to the developing roller, so as to suppress a fog phenomenon.
A development methods disclosed in each of the Patent Publications 1 to 3 is intended to reduce the number of times of a reciprocating movement of magnetic toner between the developing roller and the photoconductive drum without reducing a frequency of the AC voltage, in order to suppress the occurrence of density unevenness, positive characteristic or fog phenomenon due to a non-uniform developing magnetic field. That is, the development method is not designed to apply a developing bias voltage in such a manner as to suppress the occurrence of image unevenness due to the developing gap fluctuation, and thereby unable to solve the problem about image unevenness due to the developing gap fluctuation.
The Patent Publication 4 includes a description about an advantage of being able to suppress an image defect due to an abnormal electrical discharge which is likely to occur under a condition allowing an image to be obtained with high evenness and sufficient density, in an image forming apparatus using a two-component developing system.
This image forming apparatus is not designed to control a developing bias voltage in such a manner as to eliminate image unevenness due to the developing gap fluctuation, and thereby unable to solve the problem about image unevenness due to the developing gap fluctuation, as in the Patent Publications 1 to 3. Moreover, the image forming apparatus is based on a two-component developing system, and therefore adapted to apply a developing bias voltage to a developing roller around which a large amount of carrier exists. Therefore, even if such a developing bias voltage is applied to a developing roller around which only a few amount of carrier exists, a high-quality image cannot be obtained.